Why have capacitors improved so much over the years?


Assuming they have, which is my general impression…
redwoodaudio
Everything has improved over the years.
Defects introduce colorations.
Some prefer the colorations.
It’s market driven. Who cared about capacitors 20 or 30 years ago. People like us who tweek and upgrade electronics have found that capacitors make a difference in sound quality. Like changing tubes and interconnects.
Engineers who designed professional gear 40 and 50 years ago cared. Back then I knew some then 'old timers' who were aware. Can't comment on earlier, but no doubt some were aware.

I had a tube 'guru' tweak my Citation II with better resistors, caps & wire. Same cct & old tubes, but OMG.